Theoretical and experimental investigations of Co-Cu bimetallic alloys-incorporated carbon nanowires as an efficient bi-functional electrocatalyst for water splitting

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Abstract

Application of noble metal-free electrocatalysts for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) during electrocatalytic water splitting is crucial for clean energy conversion and has drawn extensive attention. However, the development of highly active and low cost electrocatalysts is a considerable challenge. Herein, Co-Cu alloy nanoparticles-incorporated carbon nanowires electrocatalyst was synthesized and evaluated for both OER and HER. The nanomaterials were fabricated by facile electrospinning of sol-gel composed of cobalt acetate, copper acetate, and poly(vinyl alcohol) followed by calcination in an inert environment. Adjusting the composition of the metallic counterpart was found to significantly enhance electrochemical properties of the catalyst. Furthermore, the unique nanowire morphology and structural properties of incorporated Co-Cu alloy, the (Co0.95Cu0.05@CNWs) composition exhibits good electrocatalytic performance for both OER and HER in the alkaline medium. Physicochemical characterizations using X-ray diffraction, X-ray photoelectron spectroscope, scanning electron microscopy, and transmission electron microscopy have confirmed the formation of alloy structure and nanowire morphology. The optimum composition (Co0.95Cu0.05@CNWs) requires small overpotential, ɳ10 of ∼285 mV for oxygen evolution reaction (OER) and ∼160 mV for hydrogen evolution reaction (HER) with the corresponding Tafel slope of 92 mV dec−1 and 172 mV dec−1 versus the reversible hydrogen electrode, respectively. In addition, only negligible loss in activity was observed after 1000 cycles and prodeces cell voltage of 1.58 V at current of 10 mA/cm2 and 1.72 V at current density of 50 mA/cm2 in two electrode system. Density Functional Theory (DFT) calculations were employed to verify experimental results. Electronic density of states (DOS) results reveal an increase in electronic states near the Fermi level upon Co-Cu heterojunctioning with CNWs. This is indicative of improved catalytic activity and more favorable binding energies of HER and OER intermediates. Reaction coordinate diagrams for HER and OER were developed, which aided in identifying thermodynamically limiting steps. This work may provide a feasible approach for incorporating other transition metals to design low-cost and high-performance bifunctional electrocatalysts for overall water splitting.

Introduction

World’s energy demand, fueled by population growth and improved standards of living is expected to double by 2100, however this growth should be met responsibly and sustainably manner. On this front, significant growth in renewable energy, as renewable electricity, has shown promising trends in last few years–with about 19% of electricity generated in 2019 was from wind and solar. This trend is further expected to gain momentum in future and renewables are expected to contribute about 50% of the energy mix by 2070. Thus renewable electricity generation and its use for various applications can be used to transition a fossil-dependent world to a more balanced energy portfolio with the development and deployment of renewables and its use directly, as batteries for energy storage in the residential and light duty transportation sectors, electricity storage and/or conversion to molecules such as hydrogen (H2), methanol (MeOH), ammonia (NH3) etc.

One of the very promising routes to maximize application of renewable electricity is to utilize it for production of molecular hydrogen via electrolysis of water [1]. World wide number of countries, especially European Region have laid out the aspriration to become net-zero carbon by 2050-with H2 playing a major role in achievning such a strategy. Water electrolysis is an established process that has been practiced for decades and current efforts are significaltly focused on scaling up the size of the electrolyzers in tens of megawatt for large scale production of hydrogen considered clean energy vector of the future.

Water electrolysis takes place as a combination of two reactions, cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER), as shown in Table 1 [2], [3]. The HER and OER reactions are non-spontaneous reactions and hence require the application of external energy, and use of electrocatalyst for the cathode and anode to overcome the kinetic and thermodynamic barriers [4], [5]. Noble metals such as Pt, Pd, Rh, Ir, Ru, Au, and their alloys and oxides have been widely used as electrocatalysts for both HER and OER, with Pt and Ru considered as the benchmark electrocatalyst. However, most of these metals have limited availability and supply, hence they are costly and thus there is a need to find alternates especially with significant growth expected in electrolysis in future [6]. Research on the utilization of earth-abundant and inexpensive metals, along with their different derivatives of oxides, hydroxides, and sulfides for water electrolysis has been growing rapidly in recent years [3]. Various techniques have been used to enhance overall catalytic activity, but two of the most feasible strategies are morphology control by nanostructure engineering and heteroatoms doping [7]. Nanostructure engineering of bi-metal alloys has shown higher catalytic activity in comparison to single metal catalysts [8].

Transition metals with unfilled d-orbital and numbers of unpaired d-band electrons have received considerable attention [9], [10], [11], [12]. In particular, cobalt compounds are promising candidate as electrocatalysts for HER and OER due to their wide availability and known electrocatalytic activity [13], [14], [15], [16]. Cobalt has been incorporated with other metals such as tungsten (W) in Co-W bisulfides [17], copper (Cu) in Co-Cu bimetallic [18], [19], [20], sulfides [21], [22], phosphides [23], [24], [25], [26] and nitride [19], as well as trimetalic as in Co-Ni-Cu [27]. The catalytic activity of Co-based bimetallic alloys can be enhanced by incorporation of highly conductive carbonaceous supports like graphene (G), carbon nanofibers (CNFs) and carbon nanotubes (CNTs) [28], [29], [30], [31]. Especially, carbon nanowires (CNWs) have garnered particular attention due to their high electrochemical stability, high electronic conductivity, and high surface to volume ratio [32], [33], [34], [35], [36], [37].

Inspired by the aforementioned electrochemical properties of bimetallic alloys and carbon nanostructure, a series of Co and Cu bimetallic alloys NPs-incorporated CNWs (Co0.95Cu0.05@CNWs, Co0.90Cu0.10@CNWs, Co0.85Cu0.15@CNWs, Co100Cu0.0@CNWs, Co0.0Cu100@CNWs and Co0.0Cu0.0@CNWs) were fabricated in this study by economic one-pot electrospinning of sol-gel composed of cobalt acetate, copper acetate and poly(vinyl alcohol) (PVA) followed by calcination in an inert environment. The as-synthesized nanocomposites were efficiently used as electrocatalysts for both HER and OER. It was found that the ratio between metal precursors has significant impact on overall electrocatalytic activity. The CNWs with Co0.95Cu0.05 bimetallic NPs loading (i.e. Co0.95Cu0.05@CNWs) yields good electrocatalytic activity for both HER and OER. Notably, the Co0.95Cu0.05@CNWs delivers a current density of 10 mA/cm2 at overpotentials of 285 mV for OER, and 160 mV for HER in 1.0 M KOH solution, respectively. Moreover, Co0.95Cu0.05@CNWs exhibits satisfactory overall bifunctional activity and stability. We hope that the strategy developed in this study can be used to fabricate other functional materials with incorporation of other transition metals.

Section snippets

Synthesis of CoCu@CNWs

For a typical synthesis of CoCu@CNWs, Cobalt (II) acetate tetrahydrate (Co (CO2CH3)2.4HO2), Copper (II) acetate hydrate (Cu (CO2CH3)2.xHO2) and Poly (Vinyl Alcohol) (PVA, molecular weight 650,000 g/mol)) were purchased from Sigma–Aldrich and used without any prior treatment. Prior to electrospinning, the copper and cobalt salts with various compositions (Co0.95Cu0.05, Co0.90Cu0.10, Co0.85Cu0.15, Co100Cu0.0, Co0.0Cu100) were mixed in the minimum amount of deionized water, and the resultant

Experimental

Thermal decomposition of salt acetates is a quite feasible route to synthesize metal or metal oxide catalysts [38], [39]. Calcination under controlled environment, metal acetate salts produce strong reducing gases (CO and H2), which are responsible for the complete reduction of metal acetate salts to pure metals. The main reactions occurring during the decomposition of cobalt acetate and copper acetate to pure metallic cobalt and copper are as follow [40], [41]:Co(CH3COO)2.4H2O → Co(OH) (CH3

Conclusions

In summary, we have designed CoCu bimetallic alloy nanoparticles grown on carbon nanowires for both oxygen and hydrogen evolution reactions by facile electrospinning techniques followed by calcination in an inert environment. It is found that the bimetallic CoCu alloy NPs have a significant impact on the overall electrocatalytic activity resulting in improved activity. However, among the different compositions, carbon nanowires with Co0.95Cu0.05 bimetallic nanoparticles (Co0.095Cu0.05@CNWs)

Declaration of Competing Interest

The authors report no declarations of interest.

Acknowledgments

This study is supported by a grant from the Qatar National Research Fund under National Priorities Research Program–Award Number–NPRP12S-0131-190024 and co-funding support from Qatar Shell Research and Technology Center (QSRTC). The paper’s contents are solely the responsibility of the authors and do not necessarily represent the official views of the Qatar National Research Fund.

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